A filter serves to break down organic waste, and also to provide water movement and flow. Of the various types of filters on the market, I would not go with the following designs:

Hang-On-Back filters

Hang-on filters for aquariums function well enough for many fish-only tanks. However, many of them produce an unfavourable flow pattern that changes in accordance with variation in a tank’s water level. Many of them are designed with a downward pointing rather than horizontal outflow direction, which I find less ideal. I find them usable on low tech tanks, but hard to manage on CO2 injected tanks, especially ones that depend on flow to distribute CO2 mist.​

Sponge filters powered by bubble lifts

For sponge filters in a CO2 injected planted tank, the bubble lifts off gases quite a bit of CO2. Also, unless there is supplementary flow through additional pumps, I find that most sponge filters don’t give optimal flow patterns in a tank.

Such filters also take up valuable space in the tank and are hard to hide as well. They are usable in low tech non CO2 injected tanks as a budget option, but in larger tanks, there are always more optimal choices.